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Calcite Supersaturation in Soils as a Result of Organic Matter Mineralization¹

ABSTRACT

Solutions equilibrated with soils under laboratory conditions often exhibit Ca²⁺ and CO^2-₃ activities that exceed calcite saturation. This supersaturation has been attributed to various possibilities, none of which have been fully able to explain the phenomenon. In this study we report on the attainment of calcite supersaturation in calcite + water + organic matter systems under controlled CO₂ conditions. The decomposition of alfalfa shoots (Medicago sativa L.) and soybean roots (Glycine max L. merr.) in batch reactors under CO₂ pressures of 1.0, 10.0, and 99.5 kPa was found to result in HCO₃ as an end product of aerobic decomposition. The rate of HCO₃ production from mineralizing organic matter exceeded calcite precipitation rates and, thus, calcite supersaturation resulted. The decomposition of pure organic compounds (dextrose, inositol, glucosamine, glucuronic acid, citric acid, and sodium citrate) was studied to elucidate the mechanism of HCO₃ production. It was determined that the decomposition of organic anions associated with inorganic cations always results in HCO₃ as an end product. The rates of calcite dissolution and precipitation were greatly inhibited in the presence of decomposing organic matter.

¹ Contribution from the U.S. Salinity Laboratory, USDA, ARS, 4500 Glenwood Drive, Riverside, CA 92501.

² Soil Scientist and Geochemist, respectively.

Received for publication October 20, 1986.